1. Field of the Invention
This invention relates generally to epoxy resin compositions, and more particularly, to heat-curable epoxy resin compositions containing metal carboxylate curing systems which are capable of providing exceptional latency, particularly at elevated temperatures, as well as attractive cure rates, high T.sub.g and adhesive strength, and other attractive cured properties.
2. Discussion of Related Art
Epoxy resins are important commercial products which can be cured to form insoluble, infusible films, pottings, castings, adhesives, laminates, composites, and the like. When cured, they are markedly superior in their physical, chemical, and electrical properties to many other cured thermosetting resins. They exhibit low shrinkage during curing. The combination of hardness and toughness exhibited by the cured resins, their high adhesive strength, resistance to degradation by solvents and other chemicals and their electrical properties, such as dielectric constant and resistivity, are outstanding. At the same time, these properties can be varied within wide limits depending on the end use intended for the resin. Of the wide variety of hardeners, curing agents, or homopolymerization catalysts which have been used to cure polyepoxide resins, no one is suitable for all applications, and many have serious drawbacks no matter what the application.
One-part epoxy formulations, in which the epoxy resin and curing agent are stored together, are desirable for a number of reasons. Such formulations are much more convenient to use, since no mixing is required, and can also provide improved reproducibility of results through better control of stoichiometry. An additional disadvantage of two-part formulations is their short life-time, and the fact that once mixed, they must be used or discarded.
One-part thermosetting epoxy compositions, with good storage stability at room temperature and rapidly curable at elevated temperatures are well known. Many examples, however, utilize relatively expensive curing agents. Further, relatively few are capable of providing latency at elevated temperatures. Recently, the desire for latent curing agents having enhanced stability at elevated temperatures has increased. In many application areas, efforts to provide high solids epoxy formulations with low levels of volatile organic compounds often result in increased viscosity and more difficult processing. Elevated temperatures can be used to reduce viscosity and improve processibility in thermosetting epoxy compositions, however, adequate pot-life must still be maintained. Further, facile reaction at reasonable curing temperatures is also desired. For example, a crosslinkable hot-melt adhesive would require formulation stability at an elevated application temperature, but ideally could be cured at some slightly higher temperature. Thus, the desire for latency at elevated temperatures, while maintaining facile reactivity without having to go to excessively high temperatures, is placing increasingly more difficult demands on latent epoxy curing agents.
Among the curing agents for epoxy resins are those categorized as either catalytic or coreactive. Catalytic curing agents initiate resin homopolymerization, either cationic or anionic, as a consequence of using a Lewis acid or base in the curing process.
Lewis bases, such as tertiary amines, are usually employed as accelerators for other curing agents. The utility of Lewis bases for catalytic curing of polyepoxides is limited due to long cure times at high temperatures, short pot-life, and resultant relatively poor properties. Imidazoles are unique among the Lewis bases, since they are capable of providing facile curing at moderate temperatures as well as attractive properties. Their reactivity, however, tends to impart relatively poor stability to one part epoxy formulations. This is especially true when stability at elevated temperatures is required
The Lewis acid catalysts frequently employed are complexes of boron trifluoride or boron trichloride with amines or ethers. Although these complexes are capable of providing attractive latency and reactivity, they are not suitable for all applications. For example, in some applications undesirable corrosion can result.
Coreactive curing agents are polyfunctional reagents that are employed in approximately stoichiometric quantities with epoxy resins and possess active hydrogen atoms. The important classes include polyamines, polyaminoamides, polyphenols, polymeric thiols, polycarboxylic acids, and anhydrides.
Dicyandiamide is one of the most commonly employed latent epoxy curing agents. As a solid curing agent, dicyandiamide's attractive latency is due to its insolubility in epoxy resins. While it can be used alone, for improved performance dicyandiamide is generally used in combination with accelerators (e.g. ureas and imidazoles). Dicyandiamide provides one-part epoxy formulations with long term stability at room temperature and also good stability at elevated temperatures. While dicyandiamide represents the current state of the art in commercial latent curing systems, certain embodiments of the present invention can provide even better latency at high temperatures, and yet give more rapid curing at slightly higher temperatures. In other words, the temperature difference between maximum stable pot temperature and minimum cure temperature is smaller for certain embodiments of the present invention, thus providing an improved latency/reactivity profile.
In the prior art, U.S. Pat. No. 4,829,124 describes the preparation of a thermoplastic elastomer comprising a blend of (a) a carboxylated butadiene-acrylonitrile elastomer and (b) an ethylene/acrylic acid copolymer which has been partially neutralized with a metal ion, wherein the blend of (a) and (b) are dynamically crosslinked with a small amount of an epoxy having at least 2 epoxy moieties per molecule. Being thermoplastic, the product can be melted repeatedly.
U.S. Pat. No. 4,614,674 relates to the addition of a wax or wax-like substance in conjunction with divalent or trivalent metal salts or metal complexes of organic compounds as matting agents to powder coating compositions based on epoxy resins and carboxyl-terminated polyesters. Such is reported to provide a matting effect without impairing other properties of the coating composition.
U.S. Pat. No. 4,411,955 is directed to a reactive hardenable binder composition based on a polyepoxide, a polycarboxylic acid unit based on a polymeric product, and a catalyst component in the form of an alkali and/or alkaline earth metal salt of a polymeric carboxylic acid. These systems are not latent, but cure at room temperature upon mixing. U.S. Pat. No. 4,370,452 teaches a process for inducing the rapid curing of a copolyester resin, wherein the copolyester resin is reacted with a curing reaction catalyst during polymerization of the copolyester resin to activate carboxyl termini thereof prior to final compounding the material with an epoxide compound. The curing reaction catalyst is selected from quaternary ammonium halides, tertiary amines, and carboxylate salts of group I and group II metals, Fe, Sn, and Co.
U.S. Pat. No. 4,240,938 is directed to a water-soluble reactive binder mixture comprising a halogen-free polymeric carboxylic acid compound; a salt-forming substance selected from an alkali metal, alkaline earth metal and quaternary ammonium salts, organic bases and ammonia; and a polymer containing hydroxyl groups or an epoxide compound. The mixtures are very reactive, i.e., even at 0.degree. C. they undergo hardening by cross-linking.
U.S. Pat. No. 4,152,284 discloses an accelerator combination for the cross-linking of powder paints containing an acrylic resin having carboxyl groups and an epoxy resin, the accelerator consisting essentially of (a) a compound of the formula LiX wherein X represents a hydroxyl ion, the anion of hydrochloric acid, hydrobromic acid, or of a carboxylic acid, and (b) a quaternary ammonium compound.
U.S. Pat. No. 3,622,442 relates to non-woven fibrous webs which are bonded using an aqueous dispersion of a copolymer of ethylene and unsaturated carboxylic acid wherein 0% to 75% of the acid groups have been neutralized with alkali metal ions, said copolymer being cross-linked with an epoxy resin or an amino-formaldehyde resin. Once the dispersion is dried, curing proceeds at room temperature.
U.S. Pat. No. 3,268,477 is directed to a process for curing polyepoxides with polycarboxylic acids in the presence of a small amount of an oxide of a metal of Groups 2a and 2b of the Periodic Table of Elements, for example, magnesium oxide.
Japan 63-4,568 (equiv. to 57-111,315) discloses a non-hardening epoxy resin composition containing at least one epoxy group per molecule, a compound containing at least two alcoholic hydroxy groups per molecule, a metal salt of a Lewis acid, and an inorganic filler having an average diameter of 0.5-10 microns. The metals described are di/trivalent metals. Curing to a hardened product is not described. Attempts to harden Epon 828 epoxy resin using the magnesium sulfate curing agent of Example 1 with ethylene glycol were unsuccessful at their conditions of 120.degree. C. for 4 hours, as well as at 180.degree. C. for 30 minutes.
Japan 51-21,838 (equiv. to 50-1,198) discloses an epoxy resin composition blended with a hardener and an organic carboxylic acid salt of Mg, Al, Cd, Cr, Mn, Co, Ni, or Cu, agitated at 70.degree.-150.degree. C. to provide a liquid product having excellent storage stability and improved properties when hardened. Although the text is ambiguous, the hardener is stated to be the aliphatic carboxylic acid corresponding to the metal salt carboxylate.
In U.S. Pat. Nos. 4,608,434 and 4,604,452, polycyanate esters are cured by transition metal carboxylates dissolved in monohydric alcohols or alkylphenols.
Japan 50-024,396, Japan 49-099,600, Canada 734,939, France 1,517,867, U.S. Pat. No. 4,605,581, and U.S. Pat. No. 3,117,099 describe the use of transition metal carboxylates to cure epoxy resins.
The use of alkali metal or alkaline earth metal carboxylates to catalyze the curing of epoxy resins by carboxylic anhydrides is described in Poland 143,594, Japan 60-255,820, Japan 52-154,899, U.S. Pat. No. 3,578,633 and South Africa 67-04,695.
The use of chromium carboxylates to catalyze the curing of epoxy resins by imides is described in U.S. Pat. Nos. 3,962,182, 3,956,241, 3,838,101 and 3,819,746.
None of the above prior art, however, teaches or discloses the improved curing systems of this invention. In the chapter entitled "Curing Agents and Modifiers", pp. 465-550 in Epoxy Resins Chemistry and Technology, 2.sup.nd Edition, edited by C. A. May, Marcel Dekker, Inc., New York, 1988, Mika and Bauer discuss tertiary amines, metal alkoxides, quanidines, imidazoles, and other anionic initiators for epoxy curing, but do not even refer to metal carboxylates for this purpose. On p. 493, the authors teach that the cure of epoxy resins by polymerization with anionic initiators has not found wide commercial acceptance because long cure cycles are required and because the resultant cures have a low heat-distortion point (T.sub.g). Contrary to this teaching, the exceptional latency, facile curing, and excellent cured properties provided by the epoxy compositions of this invention, using the alkali and alkaline earth metal carboxylate curing systems herein described, is unexpected and surprising.